Apparatus for central recording of remote meter data by periodic and sequential meter interrogation



R. E. MILFORD Jan. 5, 1965 3364,77] ECORDING OF' REMOTE METER D EQUENTIAL METER INTERROGATION ATA 3 Sheets-Sheet 1 APPARA'I'US FOR CENTRAL R BY PERIODIC AND S Filed Dec. 8, 1960 METER .rux.

R. E. MILFORD Jarl. 5, 1965 3,164,771 APPARA'IUS FOR (ZENTRAL. RECORDING OF REMOTE METER DATA BY PERIODIC AND SEQUENTIAL METER INTERROGATION Filed D80. 8, 1960 3 Sheets-Sheet 2 R. E. MILFORD Jan. 5, 1965 APPARA'IUS FOR CENTRAL. RECORDING 0F REMOTE METER DATA BY PEZRIODIC AND SEQUENTIAL METER INTERROGATION Filed Dec. 8, 1960 5 Sheecs-Sheet 3 TINIE-S ECON D5 METER UNI T 70 m=xr METER PUL5'E GENERFITOR and hour nsolution is satisfactory in the case of common 240 Volt, 3-wire 200 ampere ower circuitry, and the corresponding period between successive interrogations is then reduced to a maximum of only fiftcen seconds. Similarly a one kilowatt hour resolution could be used, since' this is the usual -billing unit. In such case, the period between successive interrogations would be seventy five sec-' onds. At the rading center the simple binary coding signals from the meter installations are memorized and identificd magnetically, or by other recording means, and the reported successive increments of energy are totalized to provide demand readings for a suitable longcr interval, such as an hour.- type of storage is then made of the dcmand readings, idcntified as to the related meters and applicable rates, and computations are made automatically for billing purposes from such stored information at the end of the monthly billing period.

tion of one hundred, or another suitable numbcr, of meter installations represent groupings which are advantageous for automatic reading purposes, and the power-service liries themselves are independent of this and may be connected With the loads and meter installations according to any desired program. While the prior practice A further magnetic or other Although the aspects of this invention which afe believed to be novel are set forth in the appended clairns, the-details of preferred embodiments and practicesof the invention, and the further objects and advantages thereof, may be most readily comprehended through reference to the following description taken in connection with the accornpanying drawings, wherein:

FIGURE 1 portrays automatic mctcr reading and computation equipment for an electric power distribution system in which the present invcntion is practiced, the illustrations being partly in schematic and partly in block form;

FIGURE 2 depicts the wave form of periodic sweepfrequency interrogation signals applied to mcter installations in the FIGURE 1 apparatus;

FIGURE 3 comprises a set of wave forms in which intcrrogation signal frequency variations are related to electrical pulse outputs of meter installations;

FIGURE 4 is a schematic diagram of an automaticreading meter installation in which a magnetostrictive element aids in producing a distinctive digital output signalj FIGURE 5 is a schematic diagrarn of a digital automatic-reading meter installation in which a piezoelectric crystal providcs frequency selectivity;

FIGURE 6 portrays an alternative form of high*speed meter reading and computation equipment in which nieter data is converted to digital forn1 f01' remote integration and processing through use of bistable electrical devices;

FIGURE 7 is a schematic diagrarn of' an auiomaticreading meter installation in which the state of a wound URE 7 and showing a second embo' diment of that form of the invention;

FIGURE 8 graiahically repre'sents pulse interrogation signals in tirned relationship to pulse output signals of otherforms of, high-speed remote meter-reading equipment;

FIGURE readingmetcr instaillatiohS in which bit information is read outahd tr'a nsferredfowardpro cessing equipment through a shiftregister stage in each metef installation;

FIGURE 10 is a scheniatic diagrarriof a digital automatic-readiug meter installation in which bit inf )rmation is read out a1id Ja triggering output Signal for the nextsucceeding metcr is provided by chain-reactidn circuitry.

The equipment depi ited in FIGURE 1 is designed for 9 is a schematic diagram of digital automatichas been for readers to visit each household location and to record the visible integrated fegistrations of power cousumptions during the interval since the last such visit, this is entirely avoided in the FIGURE 1 equipment by automatic electric reporting of measurement conditions at each meter to memory and computation apparatus of known forms in a reading center 18 which is at a reniote lo'cation such as a utility company ofiice where customer demand and billing information is ordinarily processed. For these purposes, signalling stagcs of all meter installations in any section are coupled tog6ther, and with the reading center, by a Wired line 19, remote section apparatus 20, and a communications link 21 which may also conveniently comprise a tWo-'wire 1inc. The signalling stage 22 of mctcr installation 11 is shdwn to include carn-operated switching contacts and electrical circuit componcnts, and the associated power-responsive watthour metcr structure devoidof the usual visual register mechanisms. Bach of the othei installations is similarly constructed, and their respective signalling stages are thus serially coupled in lifie 19 indcpendently ofthe ower service.

At a remote location, such as that of section apparatus 20, a known form of master source, 24, of sweep-frequency interrogation signals develops an output of coustant-amplitude Signals which periodically vary substantially linearly in frequency across a predetermined range of frequencies. The frequencies involved are such that one hundred simple low-cost tuned filters may each respond distinctively to diflerent narrow bands 01 frequencies within the overall range of frequencies swcpt by the sorce within 1.5 seconds, the repetition rate at which the sweep frequency signals are; delivred to any section being ten times as long, i.e. 15 sconds. This is the characterist ic presented graphically in FIGURE 2, whercin the frequency vs. time plots 25 and 26 are of successive intcrrogation signals applied from this source to the signalling stages of meter installtions associated with line 19. Sectionapparatus 20 serves not only the interrogation line 19 of the illustrated section, but up to nine additional lines, 27, associated with other sections each including one hundred meters. The l5-second pen'odicity of the sweep-frequency signals is significant in the interrogations, and it is intended that each of the interrogation lines shall receive a burst 0f such signals, over a different frequency range, 'each 15 seconds. Accordingly,

where section apparatus 20 serves tensuch pair s of lines,

each is interrogated only for a different 1.5 second interval duri ng the repetition period. While source 24 delivegs ten bursts of outputsignals like that of plot 25 over the entire 15 second period, each pair of the intermgation lines, witnesses only one distinctive 1.5 second portion o f it, as controlledby a line selector 28. The latter Y electronicswitch, the operation of the selcctor bcing-appropriately synchronized With boththe;operations of the sWeep-frequency source and of nicniorizing and Computuse With a nun1ber of e1ectrii:al distribution secti oris each including one hndred .consumer electric meter installations, of which tho se identified 'by reference c haracters v 11, 12 an'd'13 'representtlie first, second and lastof one such section. Three-wire service lines, 14, ntiake' the aus-1 tomary 240-Volt, ZOO-amPere, single phase power available to the r'espectivepconsumer loads15, 16'nd 17 through thesemeter installations. 'As will beco'me apparent from descriptions following later herein, the: secingequipment inthe reading centenby sYnchronizing command signalsj applied by couplings 29, so that each section line receives only itsintended share cf thesweep signals and so that the informatioh'reported back to the reading cent'er -can be identified With each meter installationin each secti0n. Of course it will be understood that- Where a second peripd is used'either' the-length of the interrogatio'n signal couldbe loriger, or more ir'1teir0gaa tion lines could bc utilized in th'csection. .Alternatively,

of coursc,;tM sweep-frequcncy rangemay;be caiuse'd to and 13 over lines 19 and with*other me ter installations over lines 27 also cooperate with the reading center 18 in =the samc manuer by way cf links 21 The signalling'stages of the metcr installations may amount as it is paralleled with it upon closure of chi:

switch contacts 33 and 34. Alternatively these switch contacts may function to short out a portion cf inductance winding 37, for the same purpose. The eflect of such detuning is to cause the impedance curve, and consequently the attendant voltagc characteristic, to occur within either the lower or higher half cf any frcquency band, such as that bctween frequencies f and f in FIG- URE 3, the peak responses in the related curve 47 being sharpened by use f high-Q tuned circuitry and the peak levels of the responses being substantially the same. Whether fthe peaked response of any 1nctcr installation occurs Within the fii'st er second half of the related nar- 1'0W band cf frequencies then characterizes whethet the meter-operated switch is in the one 01' the other of its open and closed states. This simple binary-coded information is detected in accordance With either the existing frequency or With the time of its occurrenc6, which is also related to the sweep frequency. The coding pa fern appe-aring in plot 47, FIGURE 3, is based upon downward frequency detuning of each signalling -stage as its associated switching contacts are closed, whereby the signalling information is in agreement with mhat of the co-ordinated plot 44 in the same figure where the lower level pulses represcnt a resistivc shunting cf the tuncd circuits.

The tuned circui-t elements of meter signallin'g stages may advantageously be of constructions 0ther than the simple paralleled or series capacitance and inductance already discussed. In FIGURE 4, for'example, the disk 43 of a schematirially-illustrated induction watthour meter unit 49 used with a 2-wire service' 50 drivss a triggring cam 51 through coupling 52 t closc switch 53 when each successive 200 watthour increment of energy is delivered to load 54. Switch 53 -is shown in thesimple form of an impact switch in which a flexible switch blade 53a can momentarily contact another switch elcment 53b when it drops frorn the cam projcction and springs over-center momentarily. Bach switch closure connects power from scrvice lines 50 to one winding 55 of a double solcnoid 56 over lines 57, and thereby causes the solenoid slug 58 to be drawn in one direction such that the other winding 59 maintains a low impedance. A: its distinctive resouance ;Erequency the impedance 0f a magnetostrictive ferrite core winding 60 in parallel with winding 59 oficrs a high impe clancc to the section interrogation line 6'1, but thc low impedance of thewinding 59 which follows each closing cf switch 53 causcs this signalling stagc toreport itslow impedance 10 the associzitcd section quipmenf. Simultaneously, howevcr, tl1e encrgy of the interrogation signal appewring in solenoid winding 59 ca'uscsthe slug to mov si.1bsequently in the opposite directibn, restoring' a high impedance ;t0' the signalling stage as it appears to thc interrogation linc. I-f i1pon' thi succeedingfintcrfpgagibn the load 54 has not d rawri 20'watthoursof energy, thc signalling st age' befiects'only the highcx impedanqe; other: wisc it displays the lpwerin'1pedance in thenanncr cX- The series resonant mode 0f the crystal is used, whereby upon occurrence of the desired frequency band within the interrogation signal range the capacitor 64 becomcs charged through diode 65. A temporary low-impedance shunting occurs across thc line 63 until capacitor 64 has -bccn charged, when tl1e impcdance is then high corre spouding fo that of thc high resistance 66. The capacitor retains its charge, and causes the signalling stage to exhibit a high peak impedance until the meter switch 67 is morinenta-rily closed by cam 68, whercupon capacitor 64 discharges through dissipating resistance 69. Cam 63, 01' its equiv:alent is actuated by disk 70 =of thc induction watthour unit 71 to impulse switch 67 to closure each timc the ccmsumcr load 72 absorbs another 200 Watthours of energy from the scrvice lines 73, Whereby 1he desircd binary coding cf wh'cther or not tlu's (or somc other) inoremcnt cf energy has been consumed since the next-prior interrogation is achievcd in terms cf. a high or low impedancc across lincs 63 as a particular narrow band of frequcncies is applied in section interrogation.

The system represcntation in FIGURE 6 likewise includes section*mctcr installations, such as 74 and 75, each of which may invclve an induction watthour unit 76 having its conductive disk 77 in driving relation to a switching dcvice '78, prefera bly in lhe form of.a switch 79 which is actuated to Closure momcntarily by a cam 84). A bistable device 81 in its signalling stage responds to =both signals applied through the switching device 78 over lines 82 and no address signals via coupling 83 frorn a tuned -filtcr 84. The bistablc device 81 may co'nveniently comprise a kriown magnctic core storage device or a bistable semiconductor unit, such as a doublc-base diode circuit. Devims of this character remain in the first o'f two distinctive states until switchcd to thc second by an impusle, which in the illusstrate;l circuitry is -a D.-C. pulse producedby switching device 7 8 utilizing die service mains 85 as a source in coujunction with a rctifier 86. Tuned filter 84 delivers readout signals to thc bistablc device 31 upon receiving a distinctive narrow band of frequencies from repetitive sweepfrequency interrogation sigrials-applied to the installation =by way of interrogation line 87 -f1 orrisection equi1ament 88. T he operation i s in thcse respccts similar to that out lined in relation to the system 0f=FIGURE 1. Upon -being addresscd by the read-out signals, the bistable device is automatically switched from its sccond state back to the first, if it had been placed in the second state by operation of the metcr switching device '78, and, in this process, yields an output pulse 10 the line 89 coupling the meter installa- "(ions with the section equipment 88. N0 such output pulse is occasioned unless thc bistable device has been changed from its -first I0 second states by meter recognition theit a prcdetermined increment of energy had abeen delivcred to the load since Ihe last such occurrence. Sweep frequency source 90 for*the intcr-rogations is in t.his system shown at the reading ccnter 91, and the repetition pcriod witnessed at each mcterinstallation is preferably of about :15 seconds. Where necessary 10 coordinate the pulseoutputslon line 89 wit'ri the applied sweep-freipency intermgation signals for purpos es of distinctively identifying the plained. Thislcons fiitutes ihe desircd cbding, which m'ay; 1

Inihe meter.installationpf FIGURE 5a piczcelcctcric quart z crysta l 62 coristitutes 'lhe wanted frec1ucriciy-s'cgisitive circuit .elc'ment, and is I ;cportioned touesonat2 at a distincve freque'ncy;in response tosweep-frequency inter- ;oga tipn signalsappea'1tiug -on section interrogtion ling: 63.

be processc'd asjkiescribedin conn ection witlfihcyFlGllllE information from each meter installation, a delay'imit 92, pulse shaper 93, and amplifier 94,i-all of known forms,

arc int erpsed before the binarycqdcd pulse da1a is telc- 7 and vice versa. Eachtecording drum is pnovided with a number cf recording and reproducing heads corresponding td the hurnber of meteriflg sections in the System, and each drum is speeded up to release its stored information at a 1i iultiplicd rate, the multiplyingfaotoy bcing equal center, triggers a pulse generator 146, also preferably transistorized to produce a subsequent pair of pi1lses of first one and then another polarity in line 147. Line 147 corresponds to interrogation line 138 and feeds the interrogatidn winding of tne next-succeeding meter installation. Each of these meter installations therefore reacts to the output of the next-preceding installation to deliver its distinctive pulse-coded output to the comrnon output line.

It should be understood that the emoodiments of this invention disclosed herein are intended to be of a descriptive rather than a limiting character and what various changes, combinations, substitutions or modifitzation may be practiced in accordance with these teachings without departing either in spirit er scope from this invention in its broader aspects.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Automatic metering apparatus coznprising a meter installation including a meter unit having an output momber the rnovements of which are related to the supply of electrical energy to a consumer load by service mains and having a predeterrnined maximum capacity for rneasuring the rate of the supply to the load, wnereby only one predetermined amount of the supply which represents an acceptable resolution in measurement ofthe supply to the load can be measured through said meter unit Within a. predetermined period, switching means actuated between two switching states by successive movements of said output mernber which are each related to the passage ofsaid predeterrnined amount of supply to the load through said meter unit, generator -meansiemote from said meter installtion producing succ essive electrical intrrogatiori signals recnrring at a periodicity equal to said predeter mined period, a comrnunication link applying said recurring interrogation signals to said meter installation,

- 1eieetricaisignalling mean3 fofxnin}; a art of said meterf installation coupled with said cornmuhications link and contfolledby said switching means exhibitiiig in said comrnunication link respo nsive to said inter rogtion signals differentelectrical o1itput characteristics when said sWitching means is in diif erent ones of saidswitching states, and means rernote from said meter instai1ation recording the changes in said output characteristics appearing in said cornmunication link.

2. Automatic metering apparatus comprising a meter installation including switching means and an integrating meter unit actuating said switching means between two switching states responsive tb Successive i'ntegrations of a predeterfnined amount of aeupply of electrical energy to a Consumer Toad from service'mains through said inined 111a3 irnrncapagiity foi1 measuring the-rate of the 1 supply to theload Whereby only one said predetermined amou nt of flie supply can bemeast1red through said rheter means between two switching states responsive to successive integrations of a predetermined amount of a supply of electrical energy to a difierent consumer load frorn service mains through said meter unit, said predetermined amount of tl1e supply representing an acceptable resolution in measurernent of tbe supply to consurner loads, and said -meter unit having a predeterrnined maximurn capacity for -measuring the rate of the supply to the loads, whereby only one said predetermined amount of the supply can be passed to any load through any one of themeter units within a predeterrnined period, generator means remote from said meter installations producing successive electrical intkarrogation signals recurring at intervals equal to said predetermined period, a communication link applying said recurring interrogation signals to said meter installations, electrical signalling means in each meter installation coupled with said communication link and com- V trolled by the switching means of the installation and exhibiting in said communication link responsive to said interrogation signals diflerent electrical output characteristics when said switching means of the installation is in one of said switching states and when said meter unit has ac-tuated said switching unit to the other of said switching states, each of said signalling'means exhibiting its output characteristics in said communication link within a distin btive portion of the intervals between successive interrogation Signals, and means coupled With said communication linkremotely frorn said meter installatiorls recording the supply to each of the consumer loads in accordance With changes in said output characteristics ap pearing in said commimication link Within said distinctive portions of the intervals between successive interrogation signals.

4. Automatic metering apparatus as setforth in claim 3 wherein said gener ating means comprises g1 sweep-frequency generator producingiecur ring interr'ogation sig- :nals having the sarne changes in frequenby with time during the succ'zessive interrogation intervals, and Wherein each of said eiectrical signalling means is tuned to exhibit its output characteristic's responsive nly to appearances of substantially onefrequency in said successive intermgation signals which is different from other frequencies in said-interrogation signals and which is diiferent frm the frequencies to which the others of said sighalling means are tuned.

5. Automatic metering apparatus as Set forth in claim 3 wherein each said meter unit comprises an electrical induction watthour meter motor having a disk rotated at an angular velocity proportional to the electrical energy supplied to the corisumer electrical load by electric service mains, wherein each said switching means .is actuated between said switching states responsive to each suceessive' rotation of said disk of the meter motor in the same installation by a predetermined angular a mount, wherein said predeteimined ar'nount ofsupply is substantilly 200 watthors, and wherein said communication '1ink com- Prises a Single wired line sonnecting the ele ctrical signalunit within a predeterrhined period, generator means remote from said mete'r installation producing suece'ssive electrical interrogation signal s recurring at a periodicity equalfto said predetermined period, a commuhicatin k link .applyii'xg said recurring interrogation si'gnals t0 said1 meter. ins'tallatiori, elect ricalsignalling rixeayns inT-.sid metet installation coupled with said commuriicatit1'.link and controlled by said switching me i;ms an'd exhibitingin said comniunication link r'esponsive tds"aid i nteljr0gati'on signals different electricl -output. characteristics when sid switching mans is in diffenentones of said switching ling means' in.said installations With said generating means andsaid reoording means. j

duction watthourrhater motor. havinga disk r0tated at states and means remote frorn said ni'eter inStallation recording the supply to the*consunin 1oad in accordance charaoteristios appearing in su 5plie i to the cpnsumer electricalload by electric Service mams, wherexn each said switc hing me2ns comPrises electrical coi1tacts opened and clos'ed in rsponse to rotzitions -of sa id disk by a p redetermihedangular amount; wherein sa1d predetermined.period is st1b stantially fifteen sec0nds; and whereinsaid "generating meanS produ ces said electrical interrogation signals periodically within successive "fift'een-second intervals 1 71Amomatie meteri'ng appziratosbornprisihg a plural- .;ity o f metex ing'tallations euch including an electrical in- 'duct1on -wzitthour meter motor hving a' 'disk rotategl at an angularveiocity proportional to theelectric'ai ener'gy 6. Au tomatif: metei-ing apparatus 'as' set' forth in claim 3 where 1n each said meter unit comprises an electrical inof two electrioal output characteristics in response to each cf applied successive electrical interrogation Signals 1':- crring at intervals equa1 to said period, means corinecting said switching means in a circuit relationship with said signalling means which preserves one of said output chatacteristics when said switching means is in one of said states and which establishes the other of said output characteristics when said switching means has been actuated to the other of said states by said disk, and means for couplii1g electrical interrogation signals recurring at said intervals from a communication link to said signalling means and for applying said output characteristics of said signalling means to the communication link.

15. Automatic metering apparatus as Set forth in c1aim 14 wherein said predetennined period and said Predetermined interval are substantially fifteen seconds, Wherein said predetermined increment is about 200 watthours, wherein said switching means comprises electrical contacts opened 01 closed in response to said rotatio ns of said disk by said predetermined angular amount, and wherein said electr ical signalling means is electrically tuned to exhibit said outpu-t characteristics to said coupling meansresponsive only t0 appearances of substantially one predetermined frequency in the interrogation signals from said coupling means.

16. Automatic metering apparatu3 as Set forth in c1aim 14 wherein said electrical signalling mea'ns comprises fre'- quency-snsitive means tuned to a predetermined frequency to introduc e a relatively high impedance into said coupling means at said predetermined frequency, and

im;edanceto said communication link at Said predeter- Y mined frquency whon said switohing means are in one cf said two switching states.

1 .17. Automatic metering apparatus as'set forth in claim '14 wherein said signalling means comprises an electrical filte'r cou1aled with said coupling means to receive the -interrogation signals from the communicat-ion link and -when said device is sWitched from one to the other of said stable conditions, means applying said filter output to sid switching means of said bistable device to lev: said bistable device insaid other stable condition, and mea'ns bi stable device through said firsf switching me ans to switch said bistable device to said one atable condition WALTERL. CARLSON, Primary Exah1iner.' FREDERICK MQSITRADER; ELI J. SAX, Examiner.

applying electrical energy to said switching means of said 15 when said first switching means is actuated between said two switching states.

18. Automatic metering apparatus comprising a p1ural ity of meter installations, each including an integrating meter unit and switching means, said meter unit comprising an electrical induction watthour meter motor having a disk rotated at an angular velocity proportional to the electrical energy supplied to a consumer electrical load from service mains through said meter unit, said switching means being actuated between two switching states responsive to successive rotation of said disk of said meter motor by a predetermined angular amount representing successive integrations of a predetermined amount of electrioal energy supplied to the load, s'aid predetermined amount of electrical energy representing an ac'ceptable resolution in measurementof electrical energy to loads, said meter having a predtermined capacity for measuring the rate of electrical energy supplied to loads whereby only one said predetermined amount of electrical energy can be passed to any load throngh any one said meter unit within a predetermined period, generator means remote from said meter installations producing successive electrical interrogation Signals recurring at intervals equal to said predetermined period, a communication link applying said signals to said rrieterinstallations, said communication link compn'sing a single wired line connecting said meter' instllatiorls with said generator means, electrical signalling means in each meter installation coupled to said communication link and controlled by said switching means 0f t he installation and exhibiting'in said cornmunication link responsive to said interrogation signals different e'lectrical output characteristics when said switching means of theinstallation is in one of said switch.ing states and when said swi-tching means is in the other 0f said switching states, each of said signalling means exhibiting its output characteristics in said communication link within a distinctiveportion of the intexval between successive interrogation signals, and means ooupled with said communicatidn link remote; from said meter installations fecording-the amount of electri'cal energy supplied to each consumer load in accordance With changes in said 'output 'charactefistics appearing in said communication link.

' References Cited by tlie Examina v UNITED STATES PATENTS,

3/33 Pratt 34634 1,9339 96 11/33 Paris 346'34 2207, 743- 7/40- Larson 346-34 2,942243 6/60 Bil2 340-182 

1. AUTOMATIC METERING APPARATUS COMPRISING A METER INSTALLATION INCLUDING A METER HAVING AN OUTPUT MEMBER THE MOVEMENTS OF WHICH UNIT HAVING AN OUTPUT MEM ELECTRICAL ENERGY TO A CONSUMER LOAD BY SERVICE MAINS AND HAVING A PREDETERMINED MAXIMUM CAPACITY FOR MEASURING THE RATE OF THE SUPPLY TO THE LOAD, WHEREBY ONLY ONE PREDETERMINED AMOUNT OF THE SUPPLY WHICH REPRESENTS AN ACCEPTABLE RESOLUTION IN MEASUREMENT OF THE SUPPLY TO THE LOAD CAN BE MEASURED THROUGH SAID METER UNIT WITHIN A PREDETERMINED PERIOD, SWITCHING MEANS ACTUATED BETWEEN TWO SWITCHING STATES BY SUCCESSIVE MOVEMENTS OF SAID OUTPUT MEMBER WHICH ARE EACH RELATED TO THE PASSAGE OF SAID PREDETERMINED AMOUNT OF SUPPLY TO THE LOAD THROUGH SAID METER UNIT, GENERATOR MEANS REMOTE FROM SAID METER INSTALLATION PRODUCING SUCCESSIVE ELECTRICAL INTERROGATION SIGNALS RECURRING AT A PERIODICITY EQUAL TO SAID PREDETERMINED PERIOD, A COMMUNICATION LINK APPLYING SAID RECURRING INTERROGATION SIGNALS TO SAID METER INSTALLATION, ELECTRICAL SIGNALLING MEANS FORMING A PART OF SAID METER INSTALLATION COUPLED WITH SAID COMMUNICATIONS LINK AND CONTROLLED BY SAID SWITCHING MEANS EXHIBITING IN SAID COMMUNICATION LINK RESPONSIVE TO SAID INTERROGATION SIGNALS DIFFERENT ELECTRICAL OUTPUT CHARACTERISTICS WHEN SAID SWITCHING MEANS REMOTE FROM SAID METER INSTALLATION RESTATES, AND MEANS REMOTE FROM SAID METER INSTALLATION RECORDING THE CHANGES IN SAID OUTPUT CHARACTERISTICS APPERAING IN SAID COMMUNICATION LINK. 